Numerous machines need protection against overload, to prevent the machines from serious damage in the event of operating irregularities. Thus, for example, overload switching devices are installed in electric motors, whereby when the current rises to a high level the motor is shut off from the power source. As another example, conveyors typically have fluid couplings with melting plug elements or the like installed in the coupling housing, whereby if the coupling fluid is overheated the plug melts and allows the fluid to flow out, thereby removing the torque transmission means. In the case of strictly mechanical force transmission, the coupling may be provided with so-called "shear bolts" which break when the allowable torque is exceeded. Also known are spring-preloaded jaw-clutch or gear couplings whereby when the admissible torque is exceeded the interengaging teeth of the coupling yield axially to interrupt the torque transmission process but still serve to prevent undisturbed coasting of the flywheel mass.
In the case of double-toggle-joint jaw crushers, pressure plates are used as overload protection means. The pressure plate has a defined weak locus where when the crusher becomes blocked the pressure plate breaks.
The known thermal overload protection means (e.g., overload switches and melting devices) have the disadvantage of slow response, such that they are incapable of protecting the operating machine in the face of sudden blocking. Thermal overload protection means applied to operating machines which machines have high flywheel mass are even less effective.
Jaw-clutch couplings, comprising two opposing clutch pieces connected form-interlockingly with the aid of spring means, are known to have the advantage of rapidly interrupting the drive train when a prescribed overload transmission torque is reached or exceeded. However, they have the disadvantage of not allowing coasting, because an inherent feature of the design is that the coupling immediately reengages, following which it will be reinterrupted. This is the so-called "chatter" effect.
Other known strictly mechanical overload protection devices, e.g., shear bolts and pressure plates, also work very rapidly and produce interruptions even in the presence of large flywheel masses which interruptions disconnect the operating machine from the drive means rapidly enough to prevent damage. However, they have the drawback that the protective means are destroyed at the location in which they have been installed. The mechanically destroyed parts must be replaced, so that the machine often cannot be restarred until lengthy work has been performed on the protection system.
Accordingly, the underlying problem of the present invention is to devise an overload protection device which responds rapidly in the fashion of known mechanical overload proreaction devices but without damage and with rapid restart capability. During the overload the machine should be isolated from the drive means continuously and without reaction, until the drive and the flywheel mass have stopped.